The flue gas of coal-fired industrial furnace has an emission characteristic that various pollutants such as SO2, NOx and Hg etc. coexist therein. In the current flue gas control technology for coal-fired industrial furnace, desulfuration, denitration and demercuration are often performed in multiple independent systems. The grading treatment requires not only a large space, a complex system, but also high costs of equipment investment and operation. Thus, a development of combined control technology of various pollutants is needed urgently.
The existing treatment techniques for furnace flue gas are as follows:
(1) Combined desulfuration, denitration, and demercuration with wet process in reaction tower. CN101306322A and CN101310836A disclose an integrated coordinated desulfuration, denitration and demercuration apparatus and a process of using spray bubbling method and preparation of absorption liquid, and the technical solution thereof is that: flue gas is bubbled through the absorption slurry in the absorption tower after being distributed by bubbling jet pipe, and the purpose of removing various pollutants simultaneously is achieved by gas-liquid contact; and the main component of absorption liquid is limestone slurry into which a strong oxidizing agent is added. CN101310834A discloses a method of using gas-liquid contact grading oxidation for desulfuration, denitration and demercuration process, and the technical solution thereof is that: first stage of desulfuration is carried out using lime/limestone-gypsum by wet process, and the residuals of SO2, NOx and Hg are dissolved by oxidation using an aqueous solution of oxidizing agent by gas-liquid contact to achieve second stage of desulfuration, denitration/demercuration. CN102091517A notes that an integrated absorption liquid for simultaneous desulfuration, denitration and demercuration and preparation method and use thereof, and its technical solution is that: SO2, NOx and Hg in the flue gas are washed and thus removed using mixed absorption liquid of nitrogen-containing reducer and additives by utilizing existing wet desulfuration facilities and improving existing alkalic desulfuration absorption liquid. CN102078761A proposes a comprehensive flue gas desulfuration, demercuration and denitration process and apparatus, and the technical solution thereof is that: flue gas first enters into ammonia reactor and reacts with ejected ammonia liquid, to remove partial nitrogen oxides, mercury and sulfur oxides, then enters into absorption tower and reacts again, to absorb residual sulfur oxides and heavy metal mercury by use of dedusting ash and alkaline liquid, and this achieves a combined removal of various pollutants. CN1923337A discloses an apparatus for removing various pollutants in flue gas simultaneously by ozone oxidation and method thereof, and its technical solution is: spraying ozone into furnace flue zone, feeding the ozone-treated furnace flue gas into lye washing tower for washing and absorbing sulfur oxides, nitrogen oxides, mercury and other substances, and adding H2S and other substances into the reservoir to stabilize mercury. All the above combined removals using wet process have problems that: they need a pulping system, the systems are complex; mercury which is in oxidation state has a secondary release in the wet desulfuration tower, etc.
(2) Combined desulfuration, denitration, and demercuration with dry process in reaction tower. CN1962034A discloses a method and apparatus for desulfuration, denitration and demercuration from furnace flue gas simultaneously, and its technical solution is: spraying an oxygen-enriched high activity absorbent which is consisted of fly ash, slaked lime and additives into a circulating fluidized bed reactor to remove various pollutants of SO2, NOx, Hg and others simultaneously. CN102500226A proposes an integrated apparatus and method for dry desulfuration, denitration and demercuration from flue gas, and its technical solution is that: an absorbent, an oxidizing additive and circulating ashes are added into a slaking humidifier to form a mixed ash, which is then sprayed into a reaction tower; SO2, NOx and Hg in flue gas are reacted with and adsorbed by corresponding substances and thus the flue gas is purified. The above dry combined removals use the method of adding liquid-phase, solid-phase oxidants into reaction system. Since they are gas-liquid, gas-solid heterogeneous reactions, there are problems of low contact reaction efficiency and a need for newly added complex spraying system, etc.
(3) Combined desulfuration, denitration, and demercuration in reaction tower based on induction technology. CN101337153A discloses an ultrasonic integrated desulfuration, denitration and demercuration method and apparatus, and its technical solution is: reacting strong oxidizing hydroxyl radicals released when ultrasonic cavitation was produced in reaction solution by ultrasonic with SO2, NOx and Hg in flue gas in a bubbling bed reactor to achieve the purpose of removal. CN102614776A proposes an apparatus and a method of desulfuration, denitration and demercuration combined with microwave induced catalysis, and its technical solution is that: flue gas passes through each reaction chamber inside the reactor which uses single-mode microwave source as heat source, and SO2, NOx and Hg in flue gas occurs oxidation-reduction reaction under the effect of catalyst and thereby generates inert substances or substances that can be easily removed, and thus a combined removal of SO2, NOx and Hg is achieved. The above combined removals based on induction technology employ methods for producing active free radicals, such as by high voltage discharge, etc., and there are problems of high cost and instability.
(4) Combined desulfuration, denitration, and demercuration based on catalytic oxidation. CN102527205A proposes a method and system for flue gas desulfuration, denitration, and demercuration based on catalytic oxidation simultaneously, and its technical solution is that: a catalytic reactor is arranged in a flue positioned in front of absorber; NO and Hg0 react with O2 in the flue gas and thereby generate NO2 and Hg2+ under the effect of the catalyst in the catalytic reactor; the oxidized flue gas feeds into an absorber containing absorption liquid and then the purpose of absorption of SO2, NO2 and Hg2+ is achieved simultaneously. The above combined removal based on catalytic oxidation method has a problem of low hydrothermal stability of the catalyst, and is easy to reduce efficiency by sulfur poisoning, etc.
For the combined removal of various pollutants in coal-fired furnace flue gas, the circulating fluidized bed semi-dry removal technology has advantages of owning shorter process flow, covering less area, good economy and high efficiency, etc., and is widely suitable for various reconstruction projects of our country with small venues. It is cost saving and gets great concern by related industries or enterprises. From a technical perspective, flue gas desulfuration technology using semi-dry process in circulating fluidized bed has been quite mature, and based on this, it is a domestic and international development trend to develop combined desulfuration, denitration and demercuration technology to achieve combined removal of various pollutants in flue gas and achieve upgrade of technology.